GTA: Blood Red

The zombies are coming out of nowhere and no one has a clue on how they are able to get back to life. Anyway, you need to focus on the task at hand and not on the details since you would need to live and talking about that won't help you. Fortunately, you got a gun to defend yourself with.
This game is about survival and if you can't do that then you would simply die. Each kill would earn you money and you can use the money to buy some upgrades for your character. The game would be harder as you make some progress.